The present invention relates to thermoplastic foams having improved stability to oxidation degradation. More particularly, the present invention relates to an improved stabilizer especially suited for use in extruded polyolefin foams.
It is previously known in the art to prepare thermoplastic objects having reduced densities by generating a cellular structure in such thermoplastic object. Such objects are referred to generically as foams. Foamed articles of substantial size and volume or extremely small size and volume are employed in numerous commercial applications such as cushioning and packaging materials. Foamed articles may consist of an open or closed cell structure. Such terms refer to the percentage of cells in the foam having direct communication with the outer surface of the object. They may be formed from any suitable thermoplastic polymer. Physical properties such as stiffness, density, dimensional stability, fabricability, etc. vary depending on the thermoplastic polymer employed to prepare the foam. Foams for use according to the present invention may be prepared by any suitable technique. A preferred process involves melting the resin; incorporating into the polymer melt a chemical agent, i.e. a blowing agent, to generate the desired cell structure; and thereafter foaming the thermoplastic resin. Foams having improved stiffness may be prepared by forming crosslinkages within the resin usually prior to or concurrently with the foaming process. Over time, the blowing agent generally escapes from the cells and is replaced by air.
A particular class of thermoplastic foams of generally low density for use mainly in nonstructural applications comprise polyolefin foams. These foams find substantial utility in the preparation of cushioned packaging and may be modified for different applications by the use of various polyolefins having differing physical properties such as density, tensile strength, tensile elongation, flexural modulus, etc.
Preferred polyolefin foams for use according to the present invention are foams of ethylenic polymers including high, low and medium density homopolymers of ethylene as well as copolymers of ethylene and copolymerizable ethylenically unsaturated comonomers. Such polyolefin foams are previously known in the art and described, for example, in U.S. Pat. Nos. 4,370,378; 3,505,249; 3,227,664; 3,102,865; 4,089,818; 3,808,300; 4,323,528; and other references.
Among the various performance standards required of a thermoplastic foam article, it is generally desirable that the foam not be deleteriously effected by atmospheric conditions over the useful life of the article. In particular, it is highly desirable that the deleterious effects of oxidation caused by the presence of atmospheric oxygen be eliminated or if not eliminated at least reduced to as low a level as possible. For example, in a cushioning application where it is desired to achieve repeated flexing of the foam structure, it is highly desirable that the cellular walls composed of extremely thin polymer films should not turn brittle or inelastic with age due to the effects of oxygen. In a foam, the large effective surface area exposed to available oxygen tends to accelerate the effect of oxidative degradation. As with any organic substance, oxidation may be found to be dependent upon environmental conditions. In particular, elevated temperatures and the presence of foreign substances which act as initiators or catalysts for oxidation processes may tend to exacerbate any deleterious effects due to the presence of oxygen. Accordingly, over long periods of time oxidation may result in decreased cushioning properties of a thermoplastic foam or structural failure of a thermoplastic foam due to excessive oxygen degradation of the cellular walls.
It is well known in the art, that the presence of certain compositions in the thermoplastic melt employed to make a foam can enhance the lubricating properties of the thermoplastic melt thereby resulting in reduced power consumption in extruding or processing of such polymer melts as well as reduced accumulations of polymer upon the die face during the preparation of extruded foamed articles. For example, a class of commonly employed lubricity additives are the stearates especially zinc stearate and calcium stearate. Disadvantageously, however, it has now been discovered that these metallic additives, particularly zinc stearate, deleteriously effect oxidation properties of the resulting foam structure. In particular, it has been found that a suitable measure of oxidation activity known as oxygen induction time is greatly decreased by the presence of zinc stearate or calcium stearate in a polyolefin foam structure. The reduction in measured oxygen induction time indicates the possibility of increased oxidative degradation of the polymer foam. Accordingly, it would be desirable to provide an improved thermoplastic foam having an enhanced ability to resist the effects of oxidation as indicated by a reduction in oxygen induction time.
In U.S. Pat. Nos. 4,415,707 and 4,342,848, there is disclosed the use of a processing modifier comprising polyvinyloctadecyl ether in a polyolefin resin such as polyethylene. The incorporation of this processing modifier resulted in the beneficial effects of improved surface properties in extruded articles, improved tensile strength and other physical properties of the resulting extruded polymer, and improved melt lubricity. Foamed articles were not prepared nor were oxygen induction studies performed on fabricated articles.